1. Field of the Invention
The present invention relates to an apparatus for measuring an interval between two edges of a pattern and, more particularly, to an apparatus for measuring a line width from an edge to another edge and the like of a pattern formed on a photomask, reticle, or semiconductor wafer for use in manufacturing of a semiconductor element.
2. Related Background Art
In recent years, a line width of a pattern drawn on a photomask, reticle, or the like to manufacture a semiconductor element such as IC, VLSI, and the like has rapidly become fine due to the demands to realize high integration and fine patterns. In association with this tendency, a high degree of accuracy is also required for the apparatus for measuring the line widths of those patterns. There has been known apparatus in which a pattern drawn on an object to be measured such as a mask, reticle, or the like is formed as an image by the optical system, the light intensity distribution of the pattern image is photoelectrically scanned, a time-sequential photoelectric signal is compared with a predetermined threshold value (slice level), and thereby measuring the line width of the pattern. In such a line width measuring apparatus, in order to obtain highly accurate measured values, it is necessary to optimize various kinds of optical conditions such as numerical apertures of the optical system (objective lens and the like), illuminating condition, wavelength of illumination light, focus condition, and the like and at the same time, an accurate slice level needs to be set. In general, when the above optical conditions are set, the slice level commensurate with these conditions can be promptly obtained by theoretical calculation. However, it has been found that even if the optical conditions and slice level were optimized as well, errors occur in the measured values due to the optical flare amount and the state of the pattern arrangement (isolated pattern, line and space, rectangle, etc.) and the magnitude of the line width. Particularly, when the line width decreases and becomes a value in the submicron range, the error of the measured value increases in dependence on the state of the pattern arrangement and value of the line width. Consequently, the accurate line width value cannot be obtained.